"Our discovery shows that significant quantities of metals are to be found in very remote galaxies that are too faint to be directly seen", said Celine Peroux, the lead author of the paper presenting the research.

The team used the high-resolution spectrograph UVES on the European Southern Observatory's Very Large Telescope in Chile to study the light emitted by a quasar located 9 billion light-years away that is partially absorbed by an otherwise invisible galaxy sitting 6.3 billion light-years away along the line of sight.

Spectral analysis shows that the galaxy consists of four times more metals than the Sun, and the data represent the first time astronomers have found such a large amount of metals in a very distant object. The observations also indicate the galaxy in question must be very dusty.

Almost all of the elements present in the universe were formed inside stars, which themselves are members of galaxies. By estimating how many stars formed over the history of the universe, scientists can estimate how much metal should have been produced.

For years, however, this apparently simple deduction has remained at odds with observational data. When astronomers have attempted to calculate the amount of metals observable in distant astronomical objects, the result always falls short of the predicted value by a factor of 10.

One potential mitigating ingredient has been the inherent bias in observations of distant galaxies, because only the largest and most active are detected, while the smaller and fainter members of the population remain uncounted. So astronomers recently began using quasars - the brightest distant objects known - as beacons in an attempt to flush out the more obscure galaxies.

Interstellar clouds of gas in galaxies, located between the quasars and Earth's telescopes, absorb parts of the light emitted by the giant cosmic beacons. The resulting spectral data lines contain dips that can be attributed to well-known elements. Thus, astronomers can measure the amount of metals present in galaxies that are otherwise invisible.

"This can best be done by high-resolution spectrographs on the largest telescopes, such as the Ultra-violet and Visible Echelle Spectrograph (UVES) on ESO's Kueyen 8.2-m telescope at the Paranal Observatory," Peroux said.

Her team studied the spectrum of the quasar SDSS J1323-0021 in detail, and found it shows clear indications of absorption by a cloud of hydrogen and metals located between the quasar and the telescope. Careful analysis of the spectrum revealed the galaxy to be four times richer in zinc than the Sun, while other metals such as iron appear to have condensed into vast clouds of dust grains.

"If a large number of such 'invisible' galaxies with high metal content were to be discovered, Peroux said, "they might well alleviate considerably the missing metals problem."

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